When Darth Baras negotiated the Treaty of Coruscant and ended the Great Galactic War, Angral was furious to be denied the chance to crush the Republic once and for all.[8] Tarnis inherited his father's hatred of the Republic as well, and was sent undercover on Coruscant by Angral as a scientist not long after the treaty.[9] The man put his scientific genius to use as part of the Mon CalamariGeneralVar Suthra's superweapon projects, where Tarnis and other scientists worked[3] to weaponize technologies developed by the brilliant DoctorNasan Godera during the war.[10] As Doctor "Eli Tarnis," it was not long before the Sith became the chief scientist for the entire division during the tense Cold War, and while he did contribute significantly to the other designs such as the Shock Drum and the Death Mark laser, he worked primarily on the Planet Prison—a weapon designed to ionize a planet's entire atmosphere, preventing any starships from leaving or entering the atmosphere and effectively blockading a planet without any additional naval firepower.[3] However, Tarnis also developed a way to combine the different weapons into something he called the Desolator, though he kept his research a secret.[11]

By 3643 BBY,[1] the Planet Prison was reaching the prototype stage, and the scientists were preparing to start trials when the Jedi MasterBela Kiwiiks arrived on Coruscant to investigate a darkness that the Jedi Council sensed on the capital.[3] Fearing that his cover would soon be exposed, Tarnis set in motion a plan to seize control of the Planet Prison and escape from Republic territory.[2]

Contacting the criminal syndicate known as Black Sun, Tarnis revealed his true identity as a Sith Lord and made an offer to Salarr, Black Sun's leader. In exchange for advanced weapons and armor[4] from the Sith Empire, acquired through Tarnis' allies in the Imperial-friendly Justicars' Brigade paramilitary group,[12] Black Sun would aid Tarnis in the elaborate deception he was about to perpetrate.[4] At Tarnis' request, Black Sun enlisted the RodianthiefVistis Garn—who often worked for Black Sun's rival, the Migrant Merchants' Guild—to steal the design files for the Republic superweapons. A team of Guild members under Garn's leadership raided a military warehouse and stole a shipment of heavy weapons in addition to the design files Tarnis had requested.[13]

While Carsen worked with Galen to distract the criminals with the Coruscant Security Force's security network, Din's apprentice—a recently elevated Jedi Knight—traveled to the Old Galactic Market with the astromech droidT7-O1 in order to recover the weapons and plans. While the duo raided the warehouse where Garn was holed up,[13] Tarnis contacted Black Sun and set the second stage of his plan in motion. Taking advantage of the preoccupation of Suthra and Galen's forces, Tarnis gave a Black Sun team inside information on the Senate Building's defenses so that they could sneak inside and "kidnap" him. The plan worked perfectly, and Tarnis disappeared from the building before security realized the criminals were there.[14]

Tarnis used Salarr's men for his own ends.

The Black Sun criminals retreated to the Coruscant Spaceport in order to serve as a distraction, while Tarnis himself took an air taxi—but instead of going to the gang's territory as planned,[14] he traveled to the territory of the Justicars so that he could prepare for the third stage of his plans.[9] While the Black Sun lieutenantZeer and his men fought the Coruscant Security Force in Docking Bay 84 to protect their "prize,"[14] Tarnis transmitted a secure message to the five other members of the Planet Prison design team. He informed his subordinates that the Jedi had discovered the project and were planning to shut it down, convincing them that the only way to save the project was to bring him the prototype deep in the Justicars' Sector. Believing their superior, the five stole the prototype from their lab and journeyed deep into the region to meet with Tarnis at the Justicars' Tower.[9]

During that time, the Knight had joined forces with Carsen to break through the kidnappers' lines and learned from Zeer that Tarnis had been taken to Black Sun Territory. General Suthra was alarmed to discover that the files stolen by Garn had contained information on all of the Republic's weapons projects, and so Masters Din and Kiwiiks departed Coruscant along with Agent Galen to secure each offworld facility.[14] Galen had first put the Knight in contact with the CSF's Special Tactical Unit, and the Jedi had traveled deep into Black Sun Territory with T7-O1 to meet up with the police officers. The Knight and SergeantNidaljo's men staged their attack on Salarr's headquarters and quickly broke through the criminals' defenses, causing the Black Sun leader to contact Tarnis in a fury. Tarnis brushed off Salarr's accusations of treachery and ordered the criminal to kill the Knight, who had just entered the room and discovered the "Doctor" talking to Salarr via holocomm. When the Jedi accused him of treason, Tarnis revealed his true identity as a Sith Lord before terminating the conversation[4] so that he could meet with his design team.[9]

"I'm departing Coruscant now, Father. The Planet Prison deploys in minutes. The Republic will have no choice but to surrender.""You return to me as a hero of the Empire, my son. Our victory is complete."

The five scientists gratefully handed control of the prototype over to Tarnis, but were horrified when their superior turned on them and drew his lightsaber, unleashing lightning from his fingertips as he cut them down. The Sith Lord made quick work of the defenseless scientists, though one man—Jonkan—was only wounded mortally and witnessed the Sith depart with the prototype.[9] Tarnis descended into the industrial sector known as the Works, where he was escorted to the ruins of the Jedi Temple by Imperial soldiers and began to set up the Planet Prison in the chambers of the Jedi Council.[2]

Jonkan managed to hold on long enough for the Jedi Knight to arrive with Carsen, and in his last moments explained that the device needed time to charge—and that it generated immense amounts of heat. Using the scientist's information, the two Jedi were able to locate the weapon in the Jedi Temple and raced to the ruins, cutting through the Imperials who attempted to stop them.[9]

As he waited for the device to charge, Tarnis contacted his father via a secure holotransmission[2] threaded through Ord Mantell and several other planets,[15] and informed both Angral and his father's three apprentices—Sadic, Praven, and Nefarid—of his success. Angral was greatly pleased by his son's accomplishments, and was in the process of praising Tarnis when the Knight and Carsen stormed into the room. Sadic mocked Tarnis for leaving loose ends, but Angral urged his son not to allow the Jedi to stop their victory. Refusing the Knight's offer of surrender, Tarnis summoned his red-bladed lightsaber to his hand and unleashed a storm of violet lightning at the two Jedi before charging. Though Tarnis was skilled with his blade and in the use of the dark side of the Force, he was ultimately no match for the younger Jedi's martial skill and strength in the Force, and their duel ended when the Knight struck down the Sith Lord before the unbelieving eyes of Tarnis' father.[2]

Tarnis' death drove his father to the brink of insanity, and Angral declared war upon the Republic in his effort to exact revenge upon the Jedi Knight who killed his son. Seizing control of the Republic's superweapon projects,[2] Angral used his son's research on combining superweapon technologies—Tarnis' final gift to his father—to combine the stolen weapons into the planet-killing Desolator.[15] After successfully testing the weapon on the agriworldUphrades,[16] Angral attempted to destroy the Jedi Order'shomeworld of Tython, but the Knight was able to defeat the elder Sith Lord aboared his flagshipOppressor, ending the threat of the Desolator forever.[17]

When Praven faced the Knight on the desert world of Tatooine, he understood that the Jedi's power was what ultimately led to his student's death, and the Sith was satisfied to know that his training had not been at fault.[5] Tarnis' work on the Planet Prison would later be adapted into the weapons known as the Firestorm Turbolasers by Imperial scientists during the Battle of Corellia several years later.[18]

A Human male with green eyes[3] like his father's,[17] Tarnis had brown hair and pale skin in his mild-mannered "Eli Tarnis" persona.[3] However, when Tarnis dropped the facade, the power of the dark side visibly darkened and grayed his skin, and he wore a dark hooded cloak. Tarnis was an arrogant and cruel man, and some of it bled through his disguise in that he was considered stuffy and self-important as a scientist.[3] As a Sith Lord and the son of Darth Angral, he believed himself invincible and trusted fully in the power of the dark side to destroy his enemies and ensure his victory.[2] Once he no longer had need of allies like Salarr, Black Sun,[4] and even his research team,[9] he cast them aside and continued his own plans with complete confidence that he would succeed. His arrogance stemmed from both his brilliance and his strength in the dark side, and Tarnis was willing to use both to achieve his objectives.[4]

As the son of the powerful Sith Lord Darth Angral, Tarnis inherited his father's strength in the Force[2] and was trained by Praven as a Sith.[5] In battle, Tarnis wielded a single red-bladed lightsaber with considerable skill, but relied more upon his use of Force blasts and lightning.[2] The Sith's true skills lay in subterfuge, as Tarnis was skilled enough to maintain his cover for almost a decade,[4] and in science—the man was a genius. Tarnis' work brought significant advancements to the superweapon projects,[3] and he also developed a way to combine the Shock Drum, Death Mark, and Planet Prison weapons into a single ship-mounted weapon—a feat that even the renowned Doctor Nasan Godera, the creator of those weapon technologies, acknowledged as pure brilliance.[17]

Tarnis first appeared in Star Wars: The Old Republic, a massively multiplayer online role-playing game released by BioWare and LucasArts on December 20, 2011. The ultimate antagonist of the Coruscant section of the Prologue for Jedi Knight class,[4] he is first encountered as "Doctor Eli Tarnis" in the mission "The Jedi Envoy".[3] Tarnis is the target of the missions "Kidnapped!", "Rescue", "Tracking Down the Traitor", and "Race to the Ruins", though his identity as a Sith Lord is not revealed until the end of "Rescue".[4] When the player faces Tarnis in the Jedi Temple, they can choose to ask him to surrender for light side points, or attack for dark side points, but regardless of the player's choice they will fight and kill Tarnis.[2] Tarnis is also mentioned in the 2012 reference guide Star Wars: The Old Republic Encyclopedia, in the entries on Darth Angral and the Jedi Knight class storyline.[19] In the 2015 Digital Expansion Knights of the Fallen Empire, Tarnis appears as an enemy for Jedi Knight players during the hallucination scenes in Chapter 2.[20]